The present invention relates to fuel cells. More particularly, the present invention relates to fuel cells provided with a hydrogen generation system.
Fuel cells are seen as a promising alternative to traditional power generation technologies due to their low emissions, high efficiency and ease of operation. Fuel cells operate to convert chemical energy to electrical energy. Proton exchange membrane (PEM) fuel cells comprise an anode (oxidizing electrode), a cathode (reducing electrode), and a selective electrolytic membrane disposed between the two electrodes. In a catalyzed reaction, a fuel such as hydrogen, is oxidized at the anode to form cations (protons) and electrons. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. The electrons cannot pass through the membrane, and are forced to flow through an external circuit, thus providing electrical current. At the cathode, oxygen reacts at the catalyst layer, with electrons returned from the electrical circuit, to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction product. Additionally, since the reactions are exothermic, heat is generated within the fuel cell. The half-cell reactions at the two electrodes are as follows:
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1) 
1/2O2+2H++2exe2x88x92xe2x86x92H2O+HEATxe2x80x83xe2x80x83(2) 
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a separate cooling medium. Also within the stack are current collectors, cell-to-cell seals and insulation. Piping and various instruments are externally connected to the fuel cell stack for supplying and controlling the fluid streams in the system. The stack, housing, and associated hardware make up the fuel cell unit.
Various types of fuel cells have been developed employing a broad range of reactants. For example, proton exchange membrane (PEM) fuel cells are one of the most promising replacements for traditional power generation systems. PEM fuel cells comprise an anode, a cathode, and a proton exchange membrane disposed between the two electrodes. Preferably, PEM fuel cells are fuelled by pure hydrogen gas, as it is electrochemically reactive and the by-products of the reaction are water and heat. However, these fuel cells require external supply and storage devices for the hydrogen. Hydrogen can be difficult to store and handle, particularly in non-stationary applications. Conventional methods of storing hydrogen include liquid hydrogen, compressed gas cylinders, dehydrogenation of compounds, chemical adsorption into metal alloys, and chemical storage as hydrides. However, such storage systems tend to be hazardous, dangerous, expensive and bulky.
Other types of fuels have been proposed, including hydrogen-containing materials such as methanol. In some conventional systems, external reformers are employed to liberate hydrogen from the hydrogen-containing materials. The liberated hydrogen is then introduced into the fuel cell. However, the use of external reformers complicates the construction of the system, and results in a substantial loss in system efficiency. In other conventional systems, hydrogen-containing fuels may be supplied directly to the fuel cells, i.e. supplied unreformed to the fuel cell anodes. Once inside the fuel cell, the hydrogen-containing fuel may be directly oxidized or internally reformed, and subsequently oxidized to generate electricity. This occurs in some high temperature fuel cells, such as solid oxide fuel cells. These systems do not require a separate external reformer, and utilize fuels that are easier to handle than hydrogen. However, pure hydrogen typically offers better performance, and is generally more environmentally friendly than most hydrogen-containing fuels. Moreover, high temperature fuel cells operate at a minimum temperature of 600xc2x0 C. These high temperatures are required to reform the hydrogen-containing materials prior to carrying out the fuel cell reactions. As such, hydrogen-containing materials are generally unsuitable for conventional PEM fuel cells that typically operate around 80xc2x0 C.
Another method of generating and storing hydrogen has been recently proposed. This method uses a chemical hydride solution, such as NaBH4, as a hydrogen storage medium. Generally, chemical hydride reacts with water in the presence of a catalyst to generate hydrogen, as shown in the equation below:
NaBH4+2H2Oxe2x86x924H2+NaBO2+HEATxe2x80x83xe2x80x83(3) 
The chemical hydride solution acts as both the hydrogen carrier and the storage medium. Ruthenium, Cobalt, Platinum or any alloys thereof may be used to catalyze the above reaction. It is noted that hydrogen is liberated from both the borohydride solution and the water. The borohydride solution is relatively cheap, and is much easier and safer to handle and transport than liquid or pressurized hydrogen. As a result, there are some advantages associated with using borohydride as a method of storing hydrogen as a fuel for use in fuel cells.
There are several conventional hydrogen generation systems that utilize chemical hydrides. One type of hydrogen generation system comprises a closed vessel for mixing chemical hydride powder together with water. The water is introduced into the vessel through an inlet. The vessel contains a mechanical stirring device to ensure adequate contact between the powder and the water, and to prevent the powder from clumping. The hydrogen gas is removed through an outlet in the vessel, and is supplied directly to the fuel cell. These systems tend to be inefficient since the stirring mechanism consumes energy, and increases the overall weight and complexity of the system. Furthermore, the noise generated by the stirring is undesirable. In addition, the reaction rate tends to be low, making the hydrogen generation unpredictable and thus hard to control. The systems also tend to be large and cumbersome.
Another type of hydrogen generation system employs a chemical hydride solution. In this system, an aqueous chemical hydride solution is introduced into a catalyst bed to generate hydrogen for use in fuel cells. However, these chemical hydride systems still require a separate hydrogen generation subsystem for generating and supplying hydrogen to the fuel cell system. As such, the systems tend to be complex, costly, and inefficient.
There remains a need for a fuel cell system that utilizes pure hydrogen and that contains a compact and simple subsystem for generating the hydrogen. More particularly, such a fuel cell system should desirably be equipped to liberate hydrogen from a chemical hydride solution in view of its known properties, and subsequently utilize the pure hydrogen in a fuel cell reaction.
In accordance with the present invention, there is provided a fuel cell stack, comprising:
at least one fuel cell comprising an anode with a fuel inlet port for a hydrogen containing fuel, a cathode with an oxidant inlet port;
at least one chamber for a solution comprising a solvent and at least one chemical hydride dissolved therein, and having a chamber inlet and a chamber outlet for the solution and a catalyst within at least one chamber for catalyzing reaction of the solution to generate hydrogen.
The solution used in the system can comprise a solvent comprising water and an at least one chemical hydride comprising borohydride. The at least one chemical hydride can be in the form of MbxHy, in which M is a metal. Specifically, the at least one chemical hydride can comprise one or a combination of: NaBH4, LiBH4, KBH4, or RbH4. Alternatively, the at least one chemical hydride can comprise NH3BH3. Preferably, the solution comprises a solvent comprising water and an at least one chemical hydride comprising NaBH4 and less than 5% by weight LiBH4. Preferably, the solution further comprises a freezing point depressing agent to ensure the system works properly under low temperatures. Preferably, the freezing point depressant agent comprises glycerol. Preferably, the concentration of glycerol is between 0-5% by weight, and more preferably the concentration of glycerol is 1% by weight. Preferably, the solution further comprises an alkaline additive. The alkaline additive can comprise one or a combination of: LiOH, KOH, and NaOH. More preferably, the alkaline additive comprises 0.1% NaOH by weight.
In accordance with another aspect of the present invention, there is provided an energy system, comprising:
(a) a fuel cell stack capable of generating hydrogen internally and comprising:
at least one fuel cell having an anode with a hydrogen inlet port, a cathode including an oxidant inlet port, and at least one chamber with a chamber inlet port and a chamber outlet port, and a catalyst in each chamber for catalyzing reaction of a solution comprising a solvent and an at least one chemical hydride dissolved therein to generate hydrogen;
(b) a storage means for storing the solution;
(c) a circulation loop, at least connected to the storage means, each chamber inlet port and each chamber outlet port, for circulating the solution from the storage means through the fuel cell stack;
(d) a supplying path, connected to the hydrogen inlet port of each fuel cell anode and each chamber outlet port, for supplying hydrogen generated inside the chamber back to the fuel cell;
wherein the fuel cell stack generates electricity and water from hydrogen and an oxidant.
The energy system can further include a recovery means for recovering the water generated in the fuel cell, and supplying the recovered water to the solution during the reaction as the at least one chemical hydride is consumed in use. Preferably, the recovery means comprises a gas-liquid separator.
In accordance with a further aspect of the present invention, there is provided a method for generating and supplying hydrogen to a fuel cell, the method comprising:
(a) providing a supply of solution comprising a solvent and an at least one chemical hydride dissolved therein;
(b) supplying the solution to a catalyst in the fuel cell to catalyze the reaction of the solvent and the at least one chemical hydride to generate hydrogen;
(c) removing the solution comprising hydrogen, by-products, and unreacted solution from the fuel cell;
(d) separating the hydrogen from the solution; and
(e) delivering the generated hydrogen to the fuel cell.
The method can further comprise the steps of:
(a) recovering water from consumption of hydrogen in the fuel cell; and
(b) supplying the recovered water to the supply of the solution, to compensate for water consumed during the reaction of the solution to generate hydrogen, and to promote maintenance of concentration levels for products of the reaction at acceptable levels, thereby delaying the onset of any precipitation of products tending to limit the generation of hydrogen.
The method can additionally comprise the step of adjusting the temperature of the solution upstream of the fuel cell. For example, the temperature of the solution can be either raised or lowered upstream of the fuel cell.
The internal hydrogen generation fuel cell according to the present invention can be incorporated into a safe and compact fuel cell system, eliminating the need for bulky storage and/or separate reformer subsystems. Moreover, the chemical hydride solution stream absorbs and removes heat from the fuel cell stack. Accordingly, a separate cooling loop may no longer be required. Furthermore, the hydrogen gas may be humidified by the water vapor from the chemical hydride solution. Therefore, a separate humidification system for the anode may no longer be required. Accordingly, the system is simplified, thereby resulting in improved system efficiency and enhanced power density. Since chemical hydride reactions of the present invention can take place at subzero temperatures, the fuel cell system of the present invention can start at lower temperatures than conventional fuel cells.